On the expanded USDA map for Illinois, I am on the border between 5b and 6a. Minimum nighttime temp of -10 occasionally. I cannot speak to your particular area but it seems to me that most peonies, except for tree peonies, are hardy to zone 3 which means the ground would be frozen as much as 24 inches @ -15 to -20. I have the fernies and hybrids raised about 8 inches for drainage over my clay soil. I use a custom mix of bagged humus, sandy potting soil, and finely chopped hardwood mulch which is about 75% rotted before it is bagged. 'Black forest hardwood fines' from Lowe's.
